Bearings are critical to the reliable operation of rotors for industrial heavy frame and aeroderivative gas turbines. Their main purpose is to minimize friction between the rotor and stationary components, ensuring smooth and efficient operation under extreme conditions including to over 22,000 rpm rotating speeds, daily startups and shutdowns, idling to peak power output loads, while remaining resistant to wear and corrosion.
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Hydrodynamic thrust and journal bearings
Large heavy frame gas turbines typically call for hydrodynamic thrust and journal bearings. Typically, gas turbine design engineers will specify thrust bearings to handle axial loads generated by gas flow and mechanical forces (ensuring stable operation) and journal bearings for radial loads associated with rotor weight (minimizing friction and wear).
The number required will vary, depending on gas turbine design and size. Usually, though, designs call for two journal (one at the drive end and another at the non-drive end) and one double-acting thrust bearing. Hydrodynamic bearings work by generating a thin film of fluid, typically oil, to separate the rotor and the bearing surface. The hydrodynamic film is created by the rotation of the turbine rotor, which draws oil into the converging gap between the rotor and the bearing surface. Pressure developed in the lubricating film by the rotation of the rotor creates a wedge of oil that lifts the rotor off of the bearing surface, creating a gap filled with oil that carries the bearing load that reduces friction and wear. The shape of the surface is designed to optimize the hydrodynamic film and to dissipate heat. Designed for high reliability, hydrodynamic tilting pad bearings are used to support large gas turbines rotors.
Tilting Pad Bearings
These fluid film bearings consist of individual pads mounted on tilting mechanisms, enabling them to adapt to changes in rotor position and reduce the risk of damage and vibration. They are renowned for their reliability, durability, and ability to handle high loads, speeds, and temperatures. Tilting pad bearings are widely used for gas turbine electric power generation and oil & gas mechanical power installations..
Miba Tilting Pad Bearings
Miba is one of the leading suppliers of gas turbine bearings. Due to the tilting capability of their individual pads, Miba thrust bearings allow high sliding speeds (e.g. 160 m/s and above). With an appropriate oil supply, loads of >4 MPa are achievable for continuous operation. Self-equalizing systems for handling misalignments between the thrust collar and the bearing are also available.
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As a direct result of these operational features, Miba tilting pad bearings reduce overloads and significantly improve thrust force distribution. Another common feature is direct lubrication that reduces power loss and improves oil flow – both of which lead to lower pad temperatures. Direct lubrication can be combined with a pad oil distribution feature at the pad inlet.
Bearing maintenance and inspection guide: download here.
For an overview of MIBA bearings, please see following video! Related article: Preventing Tilting Pad Fluttering in gas turbine bearings
Playing an important role, turbine bearings is to provide support to the gas turbine rotor. These bearings ensure radial support from journal bearings and axial support from thrust bearings. Usually, the compressor bearings are put centrally in the compressor inlet casing and fastened with bolts.
Turbine engine bearings and seals are very important parts responsible for supporting the main engine rotor. The quantity of bearings required for optimal engine support is primarily required by the rotor's length and weight, which in turn are affected by the type of compressor in the engine. Engines equipped with a two-spool compressor setup typically necessitate more extensive bearing support. At a minimum, one shaft requires a deep groove ball bearing to handling both thrust and radial loads, in addition to a straight roller bearing to manage radial loads exclusively. In addition, roller bearings may be used if the shaft experiences vibration or is exceptionally long. Gas turbine rotors are typically equipmented by antifriction bearings, with both ball and roller bearings. Some new design engines utilize hydraulic bearings, where an external race is enveloped in a oil film to minimize engine-transmitted vibrations.
With many advantages, antifriction bearings are favored: they provide minimal rotational resistance, facilitate accurate alignment of rotating parts, are cost-effective and easily replace, can withstand momentary overloads, are simple to cool, lubricate, and maintain, handle both radial and axial loads, and maintain durability to let temperatures down.
The turbine bearing's main function is to support to the turbine rotor structure. It contains upper and lower shells equipped with a babbit face, and receives lubricating oil from both sides of these shells.